1. Field of the Invention
The present invention relates to a polymer scale preventive agent useful in polymerization of a monomer having an ethylenically unsaturated double bond, a polymerization vessel effective in preventing polymer scale deposition, and a process of producing a polymer using said vessel.
2. Description of the Prior Art
As a method of polymerizing a monomer having an ethylenically unsaturated double bond, there are known suspension polymerization, emulsion polymerization, solution polymerization, gas phase polymerization and bulk polymerization. In any type of the polymerizations, polymer scale is liable to be deposited on areas with which the monomer comes into contact during polymerization, such as inner walls, agitation equipment and so on of a polymerization vessel.
Deposition of the polymer scale results in disadvantages that the yield of the polymer and cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel off and mix into a polymeric product, thereby impairing the quality of formed products obtained by processing the polymeric product; and that removal of such polymer scale is extremely laborious and hence time-consuming. Further, the polymer scale contains unreacted monomers and operators may be exposed thereto, which may cause physical disorders in the operators.
Heretofore, as a method of preventing the polymer scale deposition on inner walls and so forth of polymerization vessels in the polymerization of a monomer having an ethylenically unsaturated double bond, there is known a method of coating a suitable substance as a polymer scale preventive agent on the inner walls and so forth of the polymerization vessel. As the suitable substance for the polymer scale prevention, for example, there are disclosed a special polar compound in Japanese Patent Publication (KOKOKU) No. 45-030343 (1970), a dyestuff or pigment in Japanese Patent Publication (KOKOKU) Nos. 45-030835 (1970) and 52-024953 (1977), an aromatic amine compound in Japanese Pre-examination Patent Publication (KOKAI) No. 51-050887 (1976), and a reaction product of a phenolic compound with an aromatic aldehyde in Japanese Pre-examination Patent Publication (KOKAI) No. 55-054317 (1980).
Meanwhile, where polymeric products obtained by polymerization are processed into formed products such as sheets, etc., the formed products are required to have a high whiteness. When a polymeric product is formed into a sheet or the like without adding any coloring agent, the resulting formed product is more or less colored. Such coloration is called initial coloration, which is desired to be as low as possible. Specifically, a luminosity index (L) of the formed product, given by the Hunter's color difference equation in JIS Z 8730 (1980), is required to be, for example, 70 or more in the case of vinyl chloride polymers and 80 or more in the case of SBR.
However, when a conventional polymer scale preventive agent, such as a dyestuff or pigment described in said Japanese Patent Publication (KOKOKU) Nos. 45-030835 (1970) and 52-024953 (1977), an aromatic amine compound in Japanese Pre-examination Patent Publication (KOKAI) No. 51-050887 (1976), and a reaction product of a phenolic compound with an aromatic amine in Japanese Pre-examination Patent Publication (KOKAI) No. 55-054317 (1980), is coated on the inner wall surface of a polymerization vessel to form a coating and a polymerization is carried out therein, the resulting polymeric product gives a formed product, by processing, which has a low whiteness and is inferior in initial coloration. Specifically, the above-mentioned luminosity index (L) is 65 or less in the case of the formed product of vinyl chloride polymer. This is considered as a result of the fact that many of conventional polymer scale preventive agents are colored. That is, since such colored agents may be peeled or dissolved during polymerization and mix into polymeric products, the resulting formed products will exhibit a low whiteness.
Further, said conventional polymer scale preventive agents can prevent polymer scale deposition on the areas in the liquid phase of a polymerization vessel, but have a drawback that polymer scale deposition is liable to occur at and in the vicinity of the interface between a gas phase and a liquid phase which is located at the upper section of the polymerization vessel.
Once polymer scale is deposited at or in the vicinity of the interface between the gas phase and the liquid phase, the deposited scale will grow gradually as polymerization runs are repeated, and at last peel off from the inner wall, etc., to get mixed with the polymeric product. If the polymeric product containing the polymer scale is processed into formed products such as sheets or the like, the polymer scale causes increase in fish eyes in the formed products, thereby lowering seriously the quality of the products.
Further, many of the conventional polymer scale preventive agents are toxic, which are typically shown by poisons such as aniline, nitrobenzene and formaldehyde, exemplified as a polar compound in Japanese Patent Publication (KOKOKU) No. 45-030343 (1970), and by a compound containing a heavy metal such as chromium and lead, exemplified as a pigment in Japanese Patent Publication (KOKOKU) No. 45-030835 (1970). In addition, there is a fear of carcinogenicity among some dyestuffs which are described in Japanese Patent Publication (KOKOKU) Nos. 45-030835 (1970) and 52-024953 (1977). Therefore, in the case of using such materials for polymer scale prevention, there are problems of safety and hygiene for operators.